In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs).
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically. Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) were developed within the 3rd Generation Partnership Project (3GPP). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE).
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
“4G” refers to a fourth generation of cellular wireless standards which is a successor to the second generation (2G) and third generation (3G). IMT-Advanced or “4G” (including LTE-Advanced) systems aim to provide very high peak bit rates for mobile users: up to 1 Gb/s in static and pedestrian environments and up to 100 Mb/s in high speed mobile environments. In order to achieve the performance requirements of IMT-Advanced systems, a concept known as carrier aggregation (CA) has been proposed to aggregate two or more component carriers for supporting high data rate transmissions over a wide bandwidth (i.e. up to a 100 MHz for a single UE unit), while preserving backward compatibility with legacy systems. The carrier aggregation is also called (e.g., interchangeably called) “multi-carrier system”, “multi-carrier operation”, “multi-carrier” transmission and/or reception. Typically the component carriers in carrier aggregation belong to the same technology (e.g., either all are of WCDMA or LTE). However the carrier aggregation between carriers of different technologies is also possible to increase the throughput. Using carrier aggregation between carriers of different technologies is also referred to as “multi-RAT carrier aggregation” or “multi-RAT-multi-carrier system” or simply “inter-RAT carrier aggregation”. For example, the carriers from WCDMA and LTE may be aggregated. Another example is the aggregation of LTE and CDMA2000 carriers. For the sake of clarity the carrier aggregation within the same technology may be regarded as ‘intra-RAT’ or simply ‘single RAT’ carrier aggregation.
There are two general cases or types of carrier aggregation. A first general case is continuous carrier aggregation; a second general case is non-continuous carrier aggregation. In continuous carrier aggregation the available component carriers are adjacent to each other, e.g., adjacent to one another in the same frequency band. In non-continuous carrier aggregation the aggregated component carriers are separated along the frequency band. In both cases, multiple component carriers are aggregated to serve a single user equipment unit (UE). According to existing spectrum allocation policies and the fact that the spectrum resource in the low frequency bands is scarce, it is difficult to allocate continuous 100 MHz bandwidth for a mobile network. Therefore, the non-continuous carrier aggregation technique provides a practical approach to enable mobile network operators to fully utilize their (often scattered) spectrum resources. In fact, the candidate frequency bands proposed at World Radio Conference 2007 (WRC '07) for IMT-Advanced are non-continuous and some of them are less than 100 MHz.
Non-continuous carrier aggregation are typically further categorized as (1) multiple (non-contiguous) component carriers that are separated such that they belong to different frequency bands, and (2) multiple (non-contiguous) component carriers that are within the same frequency band. Yet a third carrier aggregation category is also possible: a hybrid of contiguous and non-contiguous carriers. For example the hybrid CA may comprise of two or more adjacent carriers in one frequency band (e.g. band A) and one or more contiguous or non-contiguous carriers in another frequency band (e.g. band B).
Spectrum scarcity and high licensing costs have motivated the use of unlicensed spectrum bands, such as the industrial, scientific and medical (ISM) radio bands for communication purposes. See, e.g., ITU-R Definition of the ISM bands; http://www.itu.int/ITU-R/terrestrial/faq/index.html#g013. Indeed, the following are examples of short range “local” communication technologies utilizing unlicensed spectrum:                Bluetooth operating in the 2450 MHz band. See, e.g., http://www.bluetooth.com/English/Pages/default.aspx.        HIPERLAN, standardized for the 5800 MHz band. See, e.g., E. P. Vasilakopoulou, G. E. Karastergios and G. D. Papadopoulos, “Design and Implementation of the Hiperlan/2 Protocol”, SIGMOBILE Mobile Computing and Communications Review, Vol. 7, No. 2, pp. 20-32, ACM, New York, USA, 2003.        the IEEE 802.11 family widely deployed in the 2450 MHz and 5800 MHz bands. See, e.g., http://standards.ieee.org/getieee802/802.11.html.        
While these and some other frequency bands are free to use, certain rules and regulations concerning maximum output power, power density and so called spurious emissions must be followed. See, for example, ITU-R Regulations: http://www.itu.int/publ/R-REG-RR/e.
Although cellular systems typically operate in spectrum bands licensed to a specific cellular operator within a geographical region, operating cellular technologies in unlicensed bands have some attractive features. For example, the feasibility and main technical characteristics of operating 3GPP High Speed Packet Access (HSPA) systems in unlicensed spectrum traditionally used by IEEE 802.11 compatible wireless local area networks (WLAN) have been examined. See, e.g., Kristina Zetterberg, “High Speed Downlink Shared Channel in Unlicensed Frequency Bands”, Master Thesis, Linköpings University, 2004. Operation in unlicensed spectrum has the advantages of increased bandwidth for user data, and reduced interference in the licensed band due to steering part of the data traffic to the unlicensed band. However, since many wireless communication (and other) devices may share the same unlicensed band, interference management is crucial for obtaining acceptable performance while complying with regulatory constraints. See, e.g., “WiFi and Bluetooth Coexistence Issues and Solutions”, White Paper, Texas Instruments, http://focus.ti.com/pdfs/vf/bband/coexistence.pdf.
State of the art technologies such as Bluetooth readily support direct communication between different devices, such as mobile telephones, headsets, computer keyboards or even sensors. Recently, such direct device-to-device communications based on cellular technologies such as the 3GPP LTE system has been proposed as a means of short range (up to 100 m) communications between user equipments. The details of this technology component include the power control and interference mitigation as well as the synchronization and scheduling aspects. See, e.g., K. Doppler, M. Rinne, C. Wijting, C. B. Riberio and K. Hugl, “Device-to-device Communications as an Underlay to LTE-Advanced Networks”, IEEE Communications Magazine, pp. 42-49, Vol. 47, No. 12, December 2009.
Although the current solutions provide PHY layer mechanisms to aggregate scattered component carriers, they are designed with underlying assumption that all component carriers lie in licensed spectrum bands. When some of the components are unlicensed carriers, not only the PHY layer carrier aggregation mechanisms need to be verified, but also the implications of using mixed aggregated licensed and unlicensed bands for a single user equipment unit (UE) on radio resource management need to be revisited.
For example, due to power allocation constraints for the unlicensed carriers, the coverage area of the component carriers are necessarily different, and such difference in coverage areas has direct implications on power control, handover, and cell reselection, as well as scheduling algorithms. For instance, as illustrated in FIG. 1, the scheduler needs to be aware of a user equipment unit (UE) being in the cell center or cell edge area. FIG. 1 shows that, due to the unequal maximum power allocation (limits) in the licensed and unlicensed spectrum bands, in the downlink from the base station the coverage area includes both a cell center and a cell edge area in a mixed licensed/unlicensed aggregation scenario. There may also be a difference in uplink coverage as well. For example, a Bluetooth or WLAN device may use a maximum of 100 mW transmit power, whereas an LTE user equipment unit may use up to 250 mW. A user equipment unit UEc in the cell center may be served by the unlicensed component carriers, but a user equipment unit UEe in the cell edge area may only be served by/scheduled on the licensed component carriers.
In a mixed (i.e. mixed licensed/unlicensed) carrier aggregation scenario, deployments may differ in terms of inter-site distance (ISD) and its impact on whether continuous operation in unlicensed component carriers for a given user equipment unit (UE) is possible or not. Due to an ISD “dimensioned” for the licensed component carriers, a user equipment unit (UE) making use of mixed unlicensed/licensed carrier aggregation needs to “switch off/on” unlicensed components as they move in the coverage area. For example, FIG. 2 illustrates that the UE capable of mixed unlicensed/licensed carrier aggregation when moving from the left cell to the right cell would first be eligible for unlicensed components in the cell center area of the left cell, but would then have to switch off the unlicensed components when entering the cell edge area of the left cell and would eventually be able to switch on the unlicensed components in the cell center area of the right cell.